Notes

By default only the submit button stretches across the whole container.
- This is because the input with class .field__input has to be a DIRECT child of the .field container. This also applies to select/textarea/..
With align-items set to center, the label is always centered vertically, which looks bad with radio / textarea inputs. The solution could be to either add modifiers to field__label class or use sibling selectors see here.
With an inline form, form__submit class requires align-self set to center for vertical alignment, and any combination of align-items / justify-items on the parent does nothing.
The hidden fields should not appear within form__field or field classes or they will eat up some space if those classes define some margin.